The present invention relates to the manufacturing of image intensifier devices, and more particularly, to a process of manufacturing a photocathode.
Image intensifiers, also known as a night vision systems, multiply the amount of incident light received by the image intensifier to provide a visible image. These devices typically require some low-level residual light, such as moon or star light, in which to operate. However, the present generation of image intensifiers can also make visible the light from the near-infrared (invisible) portion of the light spectrum. As used herein, the term xe2x80x9clightxe2x80x9d means electromagnetic radiation, regardless of whether or not this light is visible to the human eye. The image intensification process involves conversion of the received ambient light into electron patterns and projection of the electron patterns onto a phosphor screen for conversion of the electron patterns into light visible to the observer. This visible light can then be viewed directly by the operator or through a lens provided in the eyepiece of the system.
Image intensifiers are constructed for a variety of applications and therefore vary in both shape and size, with proximity focused image intensifiers comprising a particular type of image intensifier having the smallest size and weight of all categories of image intensifiers. These devices are particularly useful for both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are also employed in security cameras, photographing astronomical bodies and in medical instruments to help alleviate conditions such as retinitis pigmentosis, more commonly known as night blindness. Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780, entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips issued on Jan. 28, 1992, and assigned to ITT Corporation the assignee herein.
Image intensifiers are currently manufactured in two types, commonly referred to as Generation II (GEN 2) and Generation III (GEN 3) type image intensifier tubes. The primary difference between these two types of image intensifier tubes is in the type of photocathode employed in each. Image intensifier tubes of the GEN 2 type have a multi-alkali photocathode with a spectral sensitivity in the range of 400-900 nanometers (nm). This spectral range can be extended to the blue or red by modification of the multi-alkali composition and/or thickness. GEN 3 image intensifier tubes have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity by the adsorption of cesium and oxygen on the surface. This material has approximately twice the quantum efficiency of the GEN 2 photocathode. An extension of the spectral response to the near infrared can be accomplished by alloying indium with gallium arsenide. A third type of image intensifier is current being introduced and will be know as Generation IV (GEN 4) image intensifier. A GEN 4 image intensifier is similar to the GEN 3 except improvements have been made to the microchannel plate.
A GEN 3 image intensifier tube according to the prior art is illustrated in FIG. 1. The image intensifier tube 10 comprises an evacuated envelope or vacuum housing 22 having a photocathode 12 disposed at one end of the housing 22 and a phosphor-coated anode screen 30 disposed at the other end of the housing 22. A microchannel plate 24 is positioned within the vacuum housing 22 between the photocathode 12 and the phosphor screen 30. The photocathode 12 comprises a glass faceplate 14 coated on one side with an antireflection layer 16; a aluminum gallium arsenide (AlxGa1xe2x88x92xAs) window layer 17; a gallium arsenide active layer 18; and a negative electron affinity coating 20.
The microchannel plate 24 is located within the vacuum housing 22 and is separated from the photocathode 12 by a gap 34. The microchannel plate 24 is generally made from a thin wafer of glass having an array of microscopic channel electron multipliers extending between input surfaces 26 and output surfaces 28. The wall of each channel is formed of a secondary emitting material. The phosphor screen 30 is located on a fiber optic element 31 and is separated from the output surface 28 of the microchannel plate 24 by a gap 36. The phosphor screen 30 generally includes an aluminum overcoat 32 to stop light reflecting from the phosphor screen 30 from reentering the device through the negative electron affinity coating 20.
In operation, photons from an external source impinge upon the photocathode 12 and are absorbed in the GaAs active layer 18, resulting in the generation of electron/hole pairs. The electrons generated by the photo cathode 12 are subsequently emitted into the gap 34 of the vacuum housing 22 from the negative electron affinity coating 20 on the GaAs active layer 18. The electrons emitted by the photo cathode 12 are accelerated toward the input surface 26 of the microchannel plate 24 by applying a potential applied across the input surface 26 of the microchannel plate 24 and the photo cathode 12 of approximately 800 volts.
When an electron enters one of the channels of the microchannel plate 24 at the input surface 26, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at the output surface 28 of the microchannel plate 24 and are accelerated across gap 36 toward the phosphor screen 30 to produce an intensified image. Each microscopic channel functions as a secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by applying a potential difference across the input and output surfaces of the microchannel plate 24 of about 900 volts.
Electrons exiting the microchannel plate 24 are accelerated across the gap 36 toward the phosphor screen 30 by the potential difference applied between the output surface 28 of the microchannel plate 24 and the phosphor screen 30. This potential difference is approximately 6000 volts. As the exiting electrons impinge upon the phosphor screen 30, many photons are produced per electron. The photons create an intensified output image on the output surface of the optical inverter or fiber optics element 31.
A method of manufacturing a GEN 3 image intensifier photocathode according to the prior art is illustrated in FIGS. 2a-2e. In a series of steps, a buffer layer or layers 42 are grown on a single crystal substrate 40 and an etching stop layer 44 is provided over the buffer layer 42, as illustrated in FIG. 2a. These three layers 40, 42, 44 will be subsequently removed during the formation of the faceplate/photocathode structure, and the stop layer 44 facilitates the removal of the substrate 40 during a later processing step. The substrate 40 is typically formed from GaAs, and the stop layer 44 is typically formed from a different composition than the substrate 40, such as AlxGa1xe2x88x92xAs.
As illustrated in FIG. 2b, a p-type conductivity GaAs active layer 18 is deposited on the AlxGa1xe2x88x92xAs stop layer 44. This is typically accomplished using organometallic chemical vapor deposition. After deposition of the active layer 18, an AlxGa1xe2x88x92xAs window layer 17 is deposited over the active layer 18, as illustrated in FIG. 2c. Additionally, an antireflective coating 16 can be formed over the window layer 17. This complete structure forms a photocathode wafer 12a. 
As illustrated in FIG. 2d, the photocathode wafer 12a is bonded to a faceplate 14. This is typically accomplished by heating the photocathode wafer 12a and faceplate 14 to a temperature close to the softening point of the material of the faceplate 14. This heating is followed by application of pressure to the wafer 12a and faceplate 14 to bond the wafer 12a to the faceplate 14. After cooling, the wafer 12a is rigidly bonded to the faceplate 14. As illustrated in FIG. 2e, the substrate 40, buffer layer 42, and stop layer 44 are removed by chemical etching to form the photocathode 12/faceplate 14 structure.
Subsequent steps (not illustrated) include chemical or mechanical/chemical polishing to eliminate any exposed surfaces defects, such as scratches, formed during the previous processing steps. An electrical contact layer is then applied to the outer circumference of the active layer and onto the faceplate. The photocathode is then etched a final time to remove any remaining damage to the active layer. After etching, the finished photocathode/faceplate structure is coated with negative electron affinity coating and assembled into an image intensifier tube.
Many problems are associated with the use of the above-described method of manufacturing. A number of these problems are associated with the large number, as many as 25, of processing steps involved in this method. The large number of processing steps increases the probability that defects will be introduced into one of the processes resulting in substantial loss of photocathode components during manufacturing. Many losses are caused by defects introduced into the GaAs/AlxGa1xe2x88x92xAs material during the many processing steps. For example, scratches on the GaAs surface will result in poor image quality of the image intensifier.
Another problem is the use of the high-pressure thermal bonding process to attach the photocathode to the face place. This process subjects the photocathode to stress, which degrades the crystalline quality of the photocathode material. This degradation of the photocathode material can also degrade the image quality of the image intensifier and degrade the photoresponse characteristics of the image intensifier.
Still another problem associated with this method is that the GaAs substrate material has to be removed and disposed. Not only is the material cost of the GaAs substrate substantial, this material is not even used in the finished photocathode component. Furthermore, additional costs are incurred during the disposal of toxic waste caused by arsenic contaminated chemical etching byproducts. Accordingly, a need exists for an improved method of forming a photocathode for use in an image intensifier that reduces defects imparted during the method of manufacture and eliminates the requirement of a GaAs substrate.
This and other needs are met by embodiments of the present invention which provide a method of manufacturing a photocathode on a faceplate. The method comprises forming a seed layer with a single crystal structure on the faceplate; forming a window layer over the seed layer; and forming an active layer over the window layer.
By depositing a seed layer over the faceplate and depositing a window layer over the seed layer, the present invention advantageously reduces stress between the faceplate and the photocathode materials in the window layer and the active layer. Also, this method does not require the formation and later removal of a GaAs substrate material found in prior art methods, which allows savings in costs associated with the GaAs material, processing costs of forming the GaAs substrate, and disposal and processing costs associated with removing the GaAs substrate.
A further aspect of the present invention is to clean the faceplate before the seed layer is formed. The cleaning of the faceplate, the forming of the seed layer, the forming of the window layer and the forming the of active layer are preferably performed in an organometallic chemical vapor deposition reactor system. By performing these processes within the reactor system without having to remove the faceplate and photocathode from the reactor system, the handling of the photocathode is advantageously minimized. A reduction in handling leads to reduced losses because handling of the photocathode can create defects in the photocathode material.